The invention relates to methods which provide for structures and techniques for the fabrication of wide bandgap semiconductor devices that are free of the effects of screw dislocations in the crystal substrate thereof. More particularly, the screw dislocations in the substrate crystal are displaced to predetermined locations so as to essentially eliminate the detrimental effects of the screw dislocations normally occurring in an epitaxial layer used in the formation of the wide bandgap semiconductor devices. In particular, the invention discloses methods for moving c-axis screw dislocations in SiC and GaN substrate regions to predetermined lateral locations using homoepitaxial growth.
The material properties of silicon carbide (SiC), gallium nitride (GaN), and other wide bandgap semiconductors are well known to be outstanding for solid-state power device applications that would enable more efficient power management and conversion systems and large system benefits that are not possible using today""s well-commercialized silicon solid-state devices. However, the performance and the commercialization of high-electric-field SiC power devices is well known to be severely limited by the presence of c-axis screw dislocations in the SiC substrate that, until now, have propagated into the epitaxial layers making up the SiC devices. More particularly, as described in chapter 6 of the VLSI Handbook edited by Wai-Kai Chen published by CRC Press LCC of Boco Raton, Fla., herein incorporated by reference, the best performing SiC high field devices have always been those that are small enough to fit between screw dislocations, since device performance degrades as the sizes of the devices increase to encompass more and more screw dislocations. Screw dislocations in SiC are further described in the technical paper presented at the 3rd European Symposium on X-ray Topography and High-Resolution X-ray Diffraction (X-TOP ""96), 22-24 Apr. 1996, Palermo, Italy, entitled xe2x80x9cQuantitative Analysis of Screw Dislocation in 6Hxe2x80x94SiC Single Crystalsxe2x80x9d, by M. Dudley et al, and also herein incorporated by reference. This technical article was published in the technical journal II Nuovo Cimento, Vol. 19D, No. 2-4, pp 153-164. Screw dislocations are also described in chapter 11 of the technical book entitled xe2x80x9cSemiconductor Interfaces, Microstructures and Devices: Properties and Applicationsxe2x80x9d edited by Zhe Chuan Feng and published by Institute of Physics Publishing, Bristol and Philadelphia, herein also incorporated by reference.
All commercial SiC wafers, serving as substrates, to date, contain screw dislocations distributed randomly across the substrate in average densities that are of the order of thousands per square centimeter of wafer area. All of these screw dislocations present in the wafer propagate into the epitaxial layers making up the high field devices. These screw dislocation defects are difficult to observe, and it is nearly impossible to readily predict their locations on any given wafer so that the device being fabricated cannot practically be patterned and/or placed to avoid the vast majority of these defects. This greatly harms the yield, performance, and commercialization of high beneficial SiC high-field power switching devices. It is desired to reduce or even eliminate the detrimental performance effects of screw dislocations in SiC crystals and devices.
It is a primary object of the present invention to provide a method that eliminates or reduces the detrimental effects of the screw dislocations associated with wide bandgap semiconductor substrates, particularly SiC, and involved in epitaxial layer growth and the operation of devices on wide bandgap crystal substrates.
It is another object of the present invention to displace the screw dislocations to predetermined locations that will not interfere with the desired epitaxial layer growth and device fabrication and the operation of devices on wide bandgap semiconductor substrates, particularly SiC.
Also, it is another object of the present invention to reduce the total number of screw dislocations that propagate into the epitaxial film growth on wide bandgap semiconductor substrates, particularly SiC.
It is a further object of the present invention to provide high-field wide bandgap semiconductor devices for high power conversion that do not suffer the performance degradations commonly caused by screw dislocations.
It is another object of the present invention to provide for improved alpha-SiC homoepitaxial layers to be grown on a c-axis alpha SiC substrate.
It is another object of the present invention to provide for improved seed crystals for the growth of SiC substrate boules, whereby the number of screw dislocations in the boule is reduced and/or the locations of screw dislocations are predetermined.
It is another object of the present invention to provide a more optimal distribution of screw dislocations for the growth of SiC substrate boules, whereby the average distance between screw dislocation growth stepsources can be optimized according to boule growth conditions.
Moreover, it is an object of the present invention to control the lateral position of the screw dislocations involved in the growth of epitaxial layers so as to allow wide bandgap semiconductor devices to be reproducibly patterned so as to avoid performance-degrading crystal defects normally caused by screw dislocations.
Furthermore, it is an object of the present invention to provide improved lateral epitaxial overgrowth (LEO) techniques for materials, such as Group III-nitride materials.
The present invention is directed to various methods that displace the screw dislocations to predetermined lateral locations corresponding to web convergence points of lateral growth of an epitaxial layer so that the detrimental effects of the screw dislocations are essentially eliminated from the epitaxial layers formed for wide bandgap semiconductor devices.
In one embodiment the present invention provides a method for growing at least one single crystal layer on a selected single crystal substrate having an average density of replicating nonremovable stepsource dislocations, wherein the at least one single crystal layer contains at least one replicating nonremovable stepsource dislocation confined to selected lateral point locations. The method comprises the steps of: (a) choosing a single crystal substrate material which exhibits a property that the material therein contains at least one growth plane orientation whereby under selected growth conditions the growth rate due to step-flow growth is greater than at least one hundred (100) times a growth rate due to growth involving two-dimensional nucleation; (b) preparing a planar first growth surface on the single crystal substrate that is parallel to within a predetermined angle relative to a selected crystal plane of the single crystal substrate; (c) removing material in the first growth surface so as to define at least one selected separated second growth surface with top surface area that is selected to be less than twice the inverse of said average density of replicating nonremovable stepsource dislocations in the single crystal substrate and with border shape selected to have at least one enclosed hollow region, the selected separated second growth surface defining a cumulative hollow region area enclosed by at least one interior border shape selected to obtain lateral coalescence at a selected lateral point location, wherein said cumulative hollow region area is selected to be greater than half the inverse of the average density of replicating nonremovable stepsource dislocations in the single crystal substrate; (d) treating the at least one selected separated second growth surface so as to remove any removable sources of unwanted crystal nucleation and any removable sources of steps therein; (e) depositing a homoepitaxial film on the at least one separated second growth surface under selected conditions so as to provide a step-flow growth while suppressing two-dimensional nucleation; (f) continuing the deposition of said homoepitaxial film so that said step-flow growth results and produces at least one lateral cantilevered web structure growing laterally toward the interior of said at least one enclosed hollow region; (g) continuing the deposition of the homoepitaxial film until the at least one lateral cantilevered web structure completes its lateral coalescence at a selected lateral point location thereby completely covering the at least one enclosed hollow region with at least one complete crystal roof forming at least one selected separated third growth surface of desired size and shape; and (h) continuing the deposition of the homoepitaxial film for a selected third time period until homoepitaxial film of desired vertical thickness on top of the selected separated third growth surface is achieved.